A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Electrostatic clamps may be used in lithographic apparatuses operating at certain wavelengths, e.g. EUV, since at these wavelengths, certain regions of the lithographic apparatus operates under vacuum conditions. An electrostatic clamp may be provided to electrostatically clamp an object, such as a mask or a substrate (wafer) to an object support, such as a mask table or a wafer table, respectively. An electrostatic clamping force may also be used to clamp an electrostatic clamp to a wafer table or other piece of lithographic apparatus, operating in a double-sided mode.
Conventional electrostatic clamps comprise a stack in which an electrode or plurality of electrodes are disposed between an upper (first) and a lower (second) dielectric or isolating layer. For example, if the lower layer is polished, then the electrodes are deposited on the upper polished surface. Then the upper layer is placed on top of the electrodes. The upper and lower layers are bonded. Each electrode may comprise a plurality of portions. The electrodes or electrode portions do not necessarily cover the entire surface of the lower layer. At some places no electrode may be present. The region between electrodes or electrode portions may be filled with a barrier (dielectric or insulator) layer or left empty.
The electrodes may be driven at different voltages, such as +3 kV and 0 V, or +3 kV and −3 kV, resulting in an electric field being created between different electrodes. The barriers between electrodes may be subject voltages of 3 kV or more. For example, if adjacent electrodes were driven at +3 kV and −3 kV, the barrier between the electrodes would be subject to a 6 kV potential difference.
The voltage level a barrier can withstand is a factor in its clamping performance. A failure mechanism exists whereby the barrier between electrodes fails, resulting in a discharge between electrodes, and a reduction in clamping force. Any reduction in clamping force may significantly impact the performance of the electrostatic clamp, which may result in reduced throughput of a lithography system, and reduced output of integrated circuits.